1. Field of the Invention
The present invention relates to an apparatus and method for marking and reading a small bar code inscribed on a surface of an item and more particularly, to a bar code reading system in which coaxial light is incident to and reflected by the bar code and in which the image of the bar code is magnified and projected on a screen before reading. The method for making the small bar code on the surface utilizes a conventional dicing saw blade.
2. Description of the Prior Art
There are many situations in which it is desirable to inscribe a bar code directly onto a surface of an item. For example, in the manufacturing process of silicon chips (integrated circuits), the location of a particular wafer during the manufacturing process may need to be determined. Similarly, for custom designed chips each wafer may have a different type of chip located on it, and hence, identification of a particular wafer is essential. Several methods are currently available for using bar codes to identify wafers. For example, U.S. Pat. No. 4,585,931 issued to Duncan, discloses a method for reading a bar code on a silicon wafer that utilizes radiation reflected off the bar code at a specified angle. Additionally, U.S. Pat. No. 4,585,931 discloses a method for marking the wafer with a laser and discloses a unique bar code in which the ratio of the spaces to the bars is in the range of one to four. The spacing requirements of standard bar codes are described in the publication Code 39-Alphanumeric Bar Code Specifications, published by Intermec.RTM. (Jan. 1982).
U.S. Pat. No. 4,166,574, issued to Yokoyama discloses a method for marking identification symbols on the wafer using either a laser or a diamond needle.
U.S. Pat. No. 4,095,095, issued to Muraoka, et al. discloses an apparatus for reading information contained in the semiconductor pattern as a means of identifying the wafer. This apparatus utilizes a helium neon laser source and microscope type optics and relays the scanned information to a wafer processor control apparatus.
U.S. Pat. No. 3,562,536, issued to Brunner, et al. discloses a means for identifying a semiconductor wafer in which marks etched on the wafer are illuminated from the side, reflected through microscope type optics and detected by a photosensitive element.
Other patents disclosing semiconductor wafer identification processes are U.S. Pat. No. 4,010,355, issued to Roehrman, et al.; U.S. Pat. No. 3,558,899, issued to Morgan, et al.; and U.S. Pat. No. 3,597,045, issued to Mathisen.
The use of bar codes as a means of identifying many items, e.g. silicon wafers, requires that the bar code take up as little space as possible on the wafer and that the bar code contrast sharply with the background of the wafer. In U.S. Pat. No. 4,585,931, described above, a bar code system utilizing marks which are three to four times smaller than commerically available bar codes is disclosed. The smaller characters in this new bar code are formed by using laser marking techniques. A problem with laser marking techniques is that the high temperatures generated by the laser at the surface of the wafer vaporizes some silicon and creates irregularly shaped marks. Some of the vaporized silicon cools and leaves debris which breaks off later in the process and thus, contaminating the wafer. Additionally, the high temperature of the laser etching techniques causes stress fracture to the silicon crystalline struction of the wafer.
The problem of poor contrast between the marks of the bar code and the wafer surface is addressed in U.S. Pat. No. 4,585,931 by illuminating the bar code marks with a beam of light incident on the mark at an angle selected to reduce the intensity of scattered radiation. Despite this technique, problems still remain in reading very small bar codes, particularly after films have been deposited on the wafer during the manufacturing process. Furthermore, using laser etching techniques, the smallest bar widths that can be achieved are about two mils.